This invention is directed to the preparation of light weight, hybrid laminated mirror blanks comprising a graphite or silicon carbide fiber/glass-ceramic composite body bonded to a glass or glass-ceramic facing which are especially suitable for use as laser mirror blanks.
The dispersion of inorganic fibers in ceramic, glass, and glass-ceramic bodies to produce composites demonstrating a mechanical strength and, particularly, a toughness and impact resistance which are greater than exhibited in the base body is well known to the art. Fibers of such materials as alumina, aluminosilicate, graphite, silicon carbide, and silicon nitride have been reported as being so dispersed in continuous or non-continuous fashion.
The manner in which the fibers are dispersed and the geometry and location of the fibers within the matrix significantly affect the physical properties displayed by the composite articles. For example, experience has evidenced that the distribution of the reinforcing fibers in several directions within the matrix imparts a mechanical strength, toughness, and impact resistance frequently superior to composites wherein the fibers run in a single direction.
Recently, a method has been developed for fabricating complex shapes wherein the direction and location of the reinforcing fibers can be tailored at will. The method involves the use of "prepregs"; i.e., woven or non-woven sheets of fiber reinforcement which are preimpregnated with a vehicle containing an organic binder and matrix powder. The sheets are thereafter cut into a desired design and stacked in the proper contour for the particular shape to be formed. The prepreg is then consolidated and cured into a laminated composite preform at moderate temperature and pressure. Subsequently, the preform may be repressed to a final shape.
Whether simple hot pressing is employed or the prepreg technique followed, the powdered glass which is the precursor for the glass-ceramic matrix will be crystallized either during the heat treatment inherent in the hot pressing step or in a subsequent heat treatment specifically designed to crystallize the precursor glassy matrix in situ.
U.S. Pat. No. 4,256,378 supplies a recital of prior art dealing with the construction of laser mirrors (noting U.S. Pat. Nos. 3,836,236, 3,926,510, and 3,942,880), and provides an excellent discussion of the structure of laser mirrors, along with the chemical and physical properties that the materials comprising such mirrors should exhibit. The patent mentions the prior use of highly polished metal blocks for high energy laser applications and graphite fiber reinforced resin matrix composites or low expansion glasses for low energy laser applications. However, because of deficiencies witnessed in those products, U.S. Pat. No. 4,256,378 disclosed a laser mirror consisting of graphite fibers dispersed in a glass matrix.
The preferred method described in the patent contemplated passing a tow of graphite fibers through a suspension composed of powdered glass in an organic vehicle and winding the impregnated fibers onto a rotating drum. The glass powder was of such dimensions that at least 90% passed through a 325 mesh screen (44 microns). Excess glass and solvent were removed by pressing a squeegee against the drum as it rotated, and the windings, assuming the structure of a tape, were dried to remove the organic vehicle. Thereafter, the fiber was removed from the drum and cut into strips up to the diameter of the mirror to be fabricated. The strips were then desirably stacked up in alternating 0.degree. and 90.degree. sequences and the assembled composite hot pressed, either in an inert atmosphere or under vacuum, into an integral body. The sole glass utilized in the disclosed composites was Corning Code 7740, a borosilicate glass marketed by Corning Glass Works, Corning, N.Y., having a coefficient of thermal expansion of 32.5.times.10.sup.-7 /.degree. C. When desired, additional powder and glass could be inserted between each strip as it was laid up to achieve a preferred 40-70% by volume loading of graphite fiber in the composite.
A separate laser reflecting surface was thereafter applied to the composite. The patent noted the conventional use of a chromium-gold alloy for that purpose which could be applied via such conventional methods as spraying, vapor deposition, and cathode sputtering.
U.S. Pat. No. 4,324,843 discloses the fabrication of SiC fiber reinforced glass-ceramic composite articles wherein the glass-ceramic matrix is essentially free from TiO.sub.2 and is selected from the group of base composition systems consisting of aluminosilicate, lithium aluminosilicate, and magnesium aluminosilicate. The method for fabricating such articles generally followed the steps outlined above with respect to U.S. Pat. No. 4,256,378. Hence, a tow of SiC fibers was passed through a slip composed of powdered glass of a composition suitable for crystallization in situ to a glass-ceramic, an organic solvent, and organic plasticizer and wound around a rotating spool. The glass powder was so finely divided that, preferably, at least 90% passed through a 325 mesh screen. Excess glass and solvent were removed by pressing a squeegee against the spool as it rotated, and the windings, assuming the structure of a type composed of unidirectional fiber, were dried to eliminate the organic solvent. Subsequently, the tape was removed from the spool and cut into strips to conform to the dimensions of the article desired. Those strips were then laid up in any designed sequence, e.g., 0.degree./90.degree., or 0.degree./30.degree./60.degree./90.degree., or 0.degree./.+-.45.degree./90.degree. , and the assembly hot pressed, either in an inert atmosphere or under vacuum, into a unitary body. In general, the temperature of the hot pressing operation will be such as to concurrently cause crystallization in situ of the glassy matrix.